The principle of generating a cooling effect using heat is well-known and the traditional absorption cycle is based on this principle. However, due to the many problems associated with the traditional absorption cycles attention is shifting towards adsorption. Adsorption does not require a liquid solution to operate in the cycle which simplifies it drastically and recent advancements in electronic control mechanisms make it easier to run with steady operation even with a variable heating supply.
Previously proposed uses of adsorption in air conditioning and cooling have various forms. Examples can be found in U.S. Pat. Nos. 5,806,323, 7,082,781, 4,219,341, 5,806,323, US20050103615 A1, JP2002250573, JP4291751, WO03046449, WO2004/094948 A1, CN1266168, and U.S. Pat. No. 6,170,279.
The techniques described in U.S. Pat. Nos. 5,806,323, 7,082,781, 4,219,341, JP4291751, and U.S. Pat. No. 5,806,323 are based on the principle of de-humidification of air (desiccant effect) which allows the removal of humidity from air such that evaporative cooling can be subsequently performed. Although the removal of humidity from air is an adsorption principle, it has certain problems. The desiccant effect works for humid air and cannot generate chilled water. It is not considered as an active process of cooling since evaporative cooling is needed to lower the temperature of the air in air conditioning and it cannot be used for industrial applications. Even for residential air conditioning it has two main disadvantages; the first is its inability to work in dry areas, while the second is the need to add humidity to lower the temperature of air which can become uncomfortable and water consuming.
Active adsorption machines based on silica gel and single stage operation are available in the market. These machines use water as a refrigerant and the silica gel has the ability to adsorb water vapour to generate chilled water. However, such machines suffer from the need to be air-cooled and because water at the required level of vacuum cannot be condensed at 35° C. or 40° C. ambient air temperatures, a cooling tower is needed to create a cooling water temperature below atmospheric. Such machines are also bulky in size due to the low adsorption capacity of the silica gel and consequently are expensive. WO2004/094928 discloses a modification of the silica gel adsorption chiller that operates at good efficiency and a relatively ‘low’ hot water temperature. However, the machine is still water cooled since it needs cooling temperature below 30° C. It uses a rotating core to eliminate the interchanging mechanism usually used in adsorption machines.
JP2002250573 describes a solar assisted cooling/heating device in which a solar powered adsorption chiller is used to lower the temperature of the traditional compressor based chiller in summer and provide heat for the reverse cycle heat pump in winter, thus increasing the traditional chiller efficiency. Since the adsorption chiller does need to provide low temperature and only needs to reduce the condenser temperature below atmospheric, a silica gel adsorption chiller that operates at relatively high pressure which does not need low temperature of cooling water can be used and hence no cooling tower is needed. However, that device cannot generate cooling effect by itself and it needs to be connected to a regular vapour compression refrigeration cycle to generate any required cooling.
To avoid the need for a cooling tower, a different sorbent-refrigerant pair than water-silica gel can be selected. Hence, the refrigerant can be condensed at higher temperatures than that needed in the silica gel adsorption chiller and the need for the cooling tower is eliminated. By using the same single stage cycle of the silica gel adsorption chillers, the techniques described in US20050103615, WO03046449, CN1266168, and U.S. Pat. No. 6,170,279 use adsorbents such as activated carbon, zeolite, and calcium chloride. The refrigerants utilized are ammonia and methanol. Although, these techniques use various techniques for utilising different heat sources and methods to transfer the heat, they all basically use the same cycle of operation based on a singe stage compression of the refrigerant gases in the de-sorption process. The main draw back of such cycle is that all of the adsorption of refrigerant from the adsorbent material occurs at low pressure while all of the de-sorption occurs at high pressure. Either a high temperature is needed for the de-sorption process or only low level of efficiency can be achieved. US20050103615 and WO03046449 describe systems which use concentrated solar collectors to achieve temperatures around 130° C. to get an intermediate efficiency (COP of 0.2-0.4) even for intermittent cycle (day-night cycle). U.S. Pat. No. 6,170,279 describes a system that uses the temperature of the exhaust gases from a boat engine to achieve a similar level of efficiency. By using solar energy without a concentrator the system described in CN1266168 achieves very low level of efficiency (COP 0.05-0.1).
The techniques described above suffer from the problem that they either cannot operate at ambient temperatures or operate only at low levels of efficiency. The present invention aims to overcome these problems by using a two stage compression and de-sorption process such that most of the adsorption occurs at high pressure while most of the de-sorption occurs at lower pressure. As a result the more refrigerant is adsorbed and de-sorbed at the same temperature which allows the use of a relatively low temperature source of energy whilst obtaining relatively high efficiency as compared to the existing systems. Furthermore, the present invention allows the utilization of a number of adsorbent-refrigerant pairs such that an air cooled chiller can be constructed.